Differential imaging for single-path optical wafer inspection

ABSTRACT

Methods and systems for enhanced defect detection based on images collected by at least two imaging detectors at different times are described. In some embodiments, the time between image measurements is at least 100 microseconds and no more than 10 milliseconds. In one aspect, one or more defects of interest are identified based on a composite image of a measured area generated based on a difference between collected images. In a further aspect, measurement conditions associated with the each imaged location are adjusted to be different for measurements performed by at least two imaging detectors at different times. In some embodiments, the measurement conditions are adjusted during the time between measurements by different imaging detectors. Exemplary changes of measurement conditions include environmental changes at the wafer under measurement and changes made to the optical configuration of the inspection system.

TECHNICAL FIELD

The described embodiments relate to systems for surface inspection, andmore particularly to differential imaging for optical wafer inspection.

BACKGROUND INFORMATION

Semiconductor devices such as logic and memory devices are typicallyfabricated by a sequence of processing steps applied to a substrate orwafer. The various features and multiple structural levels of thesemiconductor devices are formed by these processing steps. For example,lithography among others is one semiconductor fabrication process thatinvolves generating a pattern on a semiconductor wafer. Additionalexamples of semiconductor fabrication processes include, but are notlimited to, chemical-mechanical polishing, etch, deposition, and ionimplantation. Multiple semiconductor devices may be fabricated on asingle semiconductor wafer and then separated into individualsemiconductor devices.

Inspection processes are used at various steps during a semiconductormanufacturing process to detect defects on wafers to promote higheryield. As design rules and process windows continue to shrink in size,inspection systems are required to capture a wider range of physicaldefects on wafer surfaces while maintaining high throughput.

In general, optical wafer inspection involves collecting images of awafer and detecting irregularities in the detected images. Typically, asemiconductor wafer includes multiple die, fabricated adjacent to oneanother. In a perfect manufacturing scenario, each die are identical,and thus, the wafer image should have a perfect periodic structure.Irregularities detected in the wafer image are caused by defects in theperiodic structure of the wafer. These defects are targets of the waferinspection.

The collected wafer image is affected by noise and is therefore neverperfectly periodic. Some of the noise is introduced by imperfections ofthe wafer inspection tool. In addition, a significant source ofmeasurement noise is due to actual wafer defects that do not have asignificant impact on the functionality of the manufacturedsemiconductor device. These insignificant defects are commonly referredto as “nuisance” defects. Other defects do have a significant impact onthe functionality of the manufactured semiconductor device. Thesesignificant defects are commonly referred to as “defects of interest”(DOI).

In general, the goal of wafer inspection is to capture as many defectsof interest as possible and as few nuisance defects as possible. Overtime, wafer inspection tools have evolved in an on-going effort to boostDOI visibility and suppress nuisance defects. However, improvements towafer inspection systems are desired to gather additional signalinformation useful for improving sensitivity to DOIs, and distinguishingbetween DOIs and nuisance defects.

SUMMARY

Methods and systems for enhancing defect detection based on imagescollected by at least two imaging detectors at different times aredescribed herein. In one aspect, the area of a wafer under measurementis measured by at least two imaging detectors at different times and acomposite image is generated based on the collected images. In someembodiments, the composite image is based on a difference between thecollected images. In some embodiments, the time between imagemeasurements is at least 100 microseconds and no more than 10milliseconds.

In another aspect, the fields of view associated with each of themultiple detectors are separated on the wafer. Furthermore, each of themultiple detectors images different portions of the wafersimultaneously. In this manner, measurement throughput remains high eventhough each location on wafer is imaged at least twice.

In a further aspect, the measurement conditions associated with themeasurements of each location on the wafer are adjusted such that themeasurement conditions are different for the measurements performed byat least two imaging detectors at different times. This, in turn,enhances defect detection performance by increasing the amount of signalinformation associated with each measured location. In some embodiments,the measurement conditions are adjusted during the time betweenmeasurements by different imaging detectors. This, in turn, enhancesdefect detection performance by increasing the amount of signalinformation associated with each measured location. Exemplary changes ofmeasurement conditions include environmental changes at the wafer undermeasurement and changes made to the configuration of the inspectionsystem, e.g., optical adjustments, focus adjustments, etc.

In some embodiments, additional sets of imaging detectors are employedto improve throughput. In some embodiments, a measured area of a wafermay be moved across the fields of view of any number of differentimaging detectors to image the measured area any number of differenttimes.

In another aspect, a semiconductor wafer inspection system includes animage combining device that generates a composite image based on thesignals associated with each independent image of the measured area. Insome embodiments, the image combining device generates the compositeimage on a pixel by pixel basis to minimize memory requirements andcompute time. In preferred embodiments, a composite image includes adifference between signals associated with measurements of eachparticular location on the wafer by the at least two imaging detectors.Defect detection of some objects is enhanced based on differentialimages because the changes between images of the same location arehighlighted in a differential image.

In a further aspect, a computing system is configured to identify one ormore defects of interest on a semiconductor wafer based on a compositeimage of a measured area.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not limiting in any way. Other aspects,inventive features, and advantages of the devices and/or processesdescribed herein will become apparent in the non-limiting detaileddescription set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram illustrative of one embodiment of amultiple zone imaging inspection system 100 that may be used to performthe inspection methods described herein.

FIG. 2 is a simplified diagram illustrative of a wafer 101 imaged bymultiple imaging detectors at different times along an inspection trackin one embodiment.

FIG. 3 is a simplified diagram illustrative of a wafer 101 imaged bymultiple imaging detectors at different times along several inspectiontracks in one embodiment.

FIG. 4 is a simplified diagram illustrative of the fields of view offour imaging detectors at the surface of a wafer 101 in one embodiment.

FIG. 5 is a flowchart illustrative of one exemplary method 200 ofdetecting defects on a wafer based on a composite image determined fromimages detected at different times.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

Methods and systems for enhancing optical wafer inspection withoutsacrificing measurement throughput are described herein. In one aspect,the area of a wafer under measurement is measured by at least twoimaging detectors at different times and a composite image is generatedbased on the collected images. In some embodiments, the composite imageis based on a difference between the collected images. Multiple imagesof the same measurement area increases signal information that allowsimproved separation between DOI and nuisance defects. The time betweenmeasurements is at least 100 microseconds and no more than 10milliseconds. By maintaining a short, fixed time between imaging of aparticular wafer location with each detector, noise induced by themeasurement system itself is minimized. For example, unmodelled waferposition errors, focus errors, etc. are common among all images of aparticular wafer location, while systematic misalignments amongdifferent imaging detectors may be precisely calibrated in advance.

In another aspect, the fields of view associated with each of themultiple detectors are separated on the wafer. Furthermore, each of themultiple detectors images different portions of the wafersimultaneously. In this manner, measurement throughput remains high eventhough each location on wafer is imaged at least twice.

In a further aspect, the measurement conditions associated with themeasurements of each location on the wafer are adjusted such that themeasurement conditions are different for the measurements performed bythe at least two imaging detectors at different times. In someembodiments, the measurement conditions are adjusted during the timebetween measurements by the at least two imaging detectors. This, inturn, enhances defect detection performance by increasing the amount ofsignal information associated with each measured location.

FIG. 1 is a simplified diagram illustrative of a semiconductor waferinspection system 100 including multiple detectors in at least one novelaspect. For simplification, some optical components of the system havebeen omitted. By way of example, folding mirrors, polarizers, beamforming optics, additional light sources, additional collectors, anddetectors may also be included. All such variations are within the scopeof the invention described herein. The inspection system describedherein may be used for inspecting patterned, as well as unpatternedwafers.

As illustrated in FIG. 1, wafer inspection system 100 includes anillumination source 110. Illumination source 110 may include, by way ofexample, a laser driven plasma light source, a laser, a diode laser, ahelium neon laser, an argon laser, a solid state laser, a diode pumpedsolid state (DPSS) laser, a xenon arc lamp, a gas discharging lamp, anLED array, or an incandescent lamp. The light source may be configuredto emit near monochromatic light or broadband light.

As depicted in FIG. 1, illumination optics 112 direct illumination light111 from illumination source 110 to objective 129. In one aspect,illumination optics 112 include multiple optical zones (e.g., zones 112Aand 112B) that each generate an illumination beam directed towardobjective 129. This enables independent selection of illuminationconditions associated with each imaging detector.

Each of the illumination beams (e.g., illumination beams 113A and 113B)are spatially separated from one another at a field plane ofillumination optics 112 conjugate to the field plane at the surface ofwafer 101. In some embodiments, optical zones 112A and 112B includeshared optical elements. In some embodiments, optical zones 112A and112B include separate optical elements associated with illuminationbeams 113A and 113B. In some embodiments, illumination optics 112includes a diffractive optical element to generate the desired number ofillumination beams at the desired size and spacing. The size, number,and spacing between illumination beams may be determined by a user ormay be automatically generated by wafer inspection system 100.

As depicted in FIG. 1, objective 129 includes lenses 114, 115, 117, andbeam splitting element 116. In some embodiments, the elements ofobjective 129 are integrated into a single opto-mechanical package.Objective 129 focuses illumination beams 113A and 113B onto distinctmeasurement locations 126A and 126B, respectively. As depicted in FIG.1, measurement locations 126A and 126B are separated by distance, D.

In general, illumination beams 113A and 113B may differ from one anotherin polarization, intensity, size and shape of the spot, pupil image,etc., for example, by the use of different polarizers, filters,apertures, etc. in each zone of illumination optics 112. Also, theillumination beams incident on wafer 101 at measurement areas 126A and126B may differ from illumination beams 113A and 113B, respectively, inpolarization, intensity, size and shape of the spot, pupil image, etc.,for example, by the use of polarizers, filters, apertures, etc. locatedin the illumination path from illumination optics 112 to wafer 101.

Objective 129 collects light 119A and 119B from measurement locations126A and 126B in response to the incident illumination light and directsthe collected light to imaging detectors 121A and 121B, respectively.The collected light 119A and 119B passes through one or more collectionoptical elements 118A and 118B, respectively. In some embodiments,collection optical elements 118A and 118B include shared opticalelements. In some embodiments, collection optical elements 118A and 118Binclude separate optical elements associated with collected beams 119Aand 119B. This enables independent selection of collection conditionsassociated with each imaging detector.

In some embodiments, collection optical elements 118A and 118B includeimage filter elements that limit the range of wavelengths of lightdirected to imaging detectors 121A and 121B, respectively. The imagefilter elements may be bandpass filters and/or edge filters and/or notchfilters. In some embodiments, each of the image filter elements 118A and118B include different spectral filters and collected light 119A and119B incident on imaging detectors 121A and 121B, respectively, havedifferent spectral characteristics (e.g., different wavelength ranges).In general, collected light 119A and 119B may differ from one another inpolarization, intensity, size and shape of the spot, pupil image, etc.,for example, by the use of different polarizers, filters, apertures,etc. in collection optical elements 118A and 118B.

As depicted in FIG. 1, semiconductor wafer inspection system 100includes a wafer positioning system 102 that moves wafer 101 within thefield of view of imaging detectors 121A and 121B. In some embodiments,wafer positioning system 102 moves wafer 101 in a scanning motion in theX-direction. In this manner, the field of view of imaging detector 121Aand the field of view of imaging detector 121B move across thesemiconductor wafer in the X-direction. In the embodiment depicted inFIG. 1, the field of view of imaging detector 121A is located ahead ofthe field of view of imaging detector 121B by distance, D.

In an exemplary operational scenario, inspection begins with wafer 101entering the field of view of detector 121A. Wafer 101 is translated inthe X-direction until wafer 101 passes through the field of view ofdetector 121B. Due to the translation of wafer 101 in the X-direction bywafer positioning system 102, the locus of measurement locationsdetected within the field of view of imaging detector 121A and alsoimaging detector 121B traces a swath across the surface of wafer 101 inthe X-direction. The swath on the surface of wafer 101 is referred to asan inspection track. FIG. 2 depicts measurement location 126A within thefield of view of imaging detector 121A and measurement location 126Bwithin the field of view of imaging detector 121B at an instant in time.Inspection track, Track_(i), illustrates the swath of imaged area acrossthe surface of wafer 101 due to a single scan in the X-direction. Insome embodiments, wafer positioning system 102 repeatedly translateswafer 101 in the Y-direction, and scans wafer 101 in the X direction toinspect the entire wafer area of wafer 101.

In general, wafer 101 may be moved across the fields of view of imagingdetectors 121A and 121B in many other operational modes. In someembodiments, wafer positioning system 102 is a rotational stage systemthat rotates wafer 101 about a center of rotation and translates thecenter of rotation such that the inspection track is a spiral pattern.In some embodiments, wafer 101 is moved in a stepwise fashion. In theseembodiments, wafer 101 is moved to a position, measured, moved toanother position, measured, in a repeated manner until the area of wafer101 is imaged by both detectors 121A and 121B.

In some embodiments, additional sets of imaging detectors are employedto improve throughput. For example, FIG. 3 illustrates the measurementareas 126A-F associated with the fields of view of three sets of twoimaging detectors. The locus of measurement locations detected withinthe fields of view of a pair of imaging detectors (e.g., imagingdetectors 121A and 121B) traces a swath associated with Track_(i). Thelocus of measurement locations detected within the fields of view ofanother pair of imaging detectors traces a swath associated withTrack₁₊₁. The locus of measurement locations detected within the fieldsof view of yet another pair of imaging detectors traces a swathassociated with Track₁₊₂. All six imaging detectors collect measurementdata simultaneously as the wafer 101 is translated in the X-direction bywafer positioning system 102. In the embodiment depicted in FIG. 3,wafer positioning system 102 scans wafer 101 in the X direction one timeto inspect the entire wafer area of wafer 101.

In the embodiments described with reference to FIG. 1 and FIG. 2, ameasured area of wafer 101 is moved across the fields of view of twoimaging detectors. In general, however, a measured area of wafer 101 maybe moved across the fields of view of any number of different imagingdetectors to image a measured area of wafer 101 any number of differenttimes. FIG. 4 depicts the measurement areas 150A-D associated with thefields of view of four imaging detector zones of a multiple zone imagingdetector device at an instant in time. As depicted in FIG. 4, the locusof measurement locations detected within the fields of view of the fourimaging detectors traces a swath associated with Track_(i). In thismanner, the measured area of wafer 101 is separately imaged at fourdifferent times. In some embodiments, a multiple zone imaging detectordevice includes multiple, small imaging detector zones (e.g., singlepixel column resolution) that generate a large number of images of eachmeasured location on wafer 101. In one embodiment, a two dimensionalCMOS detector is subdivided into multiple, small imaging detector zones.Since each pixel of a CMOS detector is independently addressable, imagesare collected independently by separate detector electronics associatedwith each of the imaging detector zones.

In some embodiments, wafer inspection system 100 may include a deflector(not shown) that scans the illumination beams over the surface of wafer101 instead of, or in addition to, the movement of wafer 101 by waferpositioning system 102. In one embodiment, the deflector may be anacousto-optical deflector (AOD). In other embodiments, the deflector mayinclude a mechanical scanning assembly, an electronic scanner, arotating mirror, a polygon based scanner, a resonant scanner, apiezoelectric scanner, a galvo mirror, or a galvanometer. The deflectorscans the light beam over the specimen. In some embodiments, thedeflector may scan the light beam over the specimen at an approximatelyconstant scanning speed.

As depicted in FIG. 2, the measurement areas 126A and 126B within thefields of view of imaging detectors 121A and 121B, respectively, arecharacterized by a height, H, and a width, W. In general, H and W may beany suitable dimensions. In some examples, H extends over a relativelylarge number of image pixels and W is the width of a single pixel. Inother examples, W is the width of multiple pixels, and imaging detectors121A and 121B collect light over a two dimensional array of pixels. Insome examples, H, may extend all the way across wafer 101 (e.g., atleast 300 millimeters). In some examples, W, may also extend all the wayacross wafer 101 (e.g., at least 300 millimeters).

In some embodiments, imaging detectors 121A and 121B are charge-coupleddevice (CCD) detectors. In some embodiments, imaging detectors 121A and121B are complementary metal on silicon (CMOS) detectors.

In some embodiments, imaging detectors 121A and 121B are integrated intoa multiple zone detector device. In some other embodiments, imagingdetectors 121A and 121B are each stand-alone devices.

In some embodiments, imaging detectors 121A and 121B operate in a timedelay integration (TDI) mode to increase signal to noise ratio. In theseembodiments, each measurement location is measured by each pixel acrossthe width dimension, W, and the measured signals associated with eachmeasurement location are summed to arrive at the image signal associatedwith each particular measurement location.

In some embodiments, imaging detectors 121A and 121B operate in a stepand flash mode or scan and flash mode. In some embodiments, imagingdetectors 121A and 121B operate in a line scanning mode (e.g., field ofview having a width of one pixel scanned across the wafer).

In another aspect, a semiconductor wafer inspection system includes animage combining device that generates a composite image based on thesignals associated with each independent image of the measured area. Insome embodiments, the image combining device generates the compositeimage on a pixel by pixel basis to minimize memory requirements andcompute time.

In preferred embodiments, a composite image includes a differencebetween signals associated with measurements of each particular locationon the wafer by the at least two imaging detectors. In some embodiments,defect detection is based on a differential image of a measured area ofwafer 101, e.g., I₁-I₂, generated by subtracting signals associated withmeasurements of each particular location on the wafer by one imagingdetector from the measurements of each particular location on the waferby another imaging detector. Defect detection of some objects isenhanced based on differential images because the changes between imagesof the same location are highlighted in a differential image. Theenhancement of defect detection is particularly apparent whenmeasurement conditions, such as environmental conditions at the wafer,measurement system configuration, etc., are changed between measurementsperformed by two different imaging detectors.

FIG. 2 depicts the measurement area 126A within the field of viewassociated with imaging detector 121A and the measurement area 126Bwithin the field of view associated with imaging detector 121B at aninstant in time. At a particular instance in time, T₁, the measurementarea 126A corresponds to a particular measurement location and themeasurement area 126B corresponds to a different, spatially distinctmeasurement location. However, since each measurement location is imagedby both imaging detectors 121A and 121B at different times, there isanother instance in time, T₂, when the measurement area 126B correspondsto the same particular measurement location imaged by detector 121A attime, T₁. Thus, in this example, each location on wafer 101 isindependently imaged two different times, and the time elapsed, ΔT,between the imaging of the particular measurement location is D/V_(X).

As depicted in FIG. 1, multiple zone imaging detector device 120includes imaging detectors 121A and 121B, delay module 123, andarithmetic processing unit (APU) 132. Image signals 122 associated withthe optical images detected by imaging detector 121A are communicated todelay module 123. Delay module 123 introduces a time delay, ΔT, whichcorresponds to the time elapsed, ΔT, between the imaging of theparticular measurement location. In this manner, the images detected bydetector 121A are delayed by precisely the amount of time it takes foran image of a measurement location on wafer 101 to travel betweenimaging detectors 121A and 121B. Thus, the time delay, ΔT, is defined bythe wafer motion. Delayed image signals 124 are communicated toarithmetic processing unit 132. In addition, image signals 125associated with the optical images detected by imaging detector 121B arecommunicated to arithmetic processing unit 132.

In the embodiment depicted in FIG. 1, a composite image 127 is generatedbased on the signals 124 and 125 associated with measurements of eachmeasured location on the wafer by imaging detectors 121A and 121B,respectively. In some embodiments, it is preferred that APU 132 operateon analog image signals or amplified, analog image signals to reducedigitization noise. In some embodiments, arithmetic processing unit 132operates directly on analog image signals (e.g., charge signals)generated by imaging detectors 121A and 121B. In some of theseembodiments, a differential image is generated pixel by pixel by chargesubtraction on the chip level. In these embodiments, the accumulatedin-well charge reflects the differential signal. This dramaticallyincreases effective well size and enables the detection of differentialsignals that would not be detectable by calculating the difference oftwo digital images in a post-processing step. In some embodiments, theanalog image signals are amplified by analog isolating amplifiers beforeprocessing by APU 132. In these embodiments, analog isolating amplifiersare employed to convert charge signals to voltage signals to reducenoise, increase amplitude, enable additional analog filtering, etc.,before processing by APU 132. In some other embodiments, analog imagesignals are converted to digital signals before processing by APU 132.Although, the analog to digital conversion introduces digitizationnoise, the resulting digital signals are more stable and are more easilystored in memory, which enables the implementation of a wider range ofdata processing algorithms. Furthermore, signal processing algorithmscan be implemented programmatically by APU 132 when APU 132 operates ondigital image signals.

In some embodiments, APU 132 generates a differential image 127 thatincludes a difference between signals associated with measurements ofeach particular location on the wafer by the two imaging detectors,e.g., I₁-I₂. In some embodiments, APU 132 generates a normalizeddifferential image 127, e.g., (I₁−I₂)/(I₁+I₂). In some embodiments, APU132 generates an averaged image 127, e.g., (I₁+I₂)/2. In someembodiments, APU 132 extends the dynamic range of one or more signals,e.g., when illumination intensity associated with the field of view ofeach imaging detector differs. In one example, APU 132 extends thedynamic range of a signal by multiplying the signal by constant value,e.g., (I₁−K*I₂). In general, APU 132 may be employed to generate acomposite image based on any suitable algorithm.

In a further aspect, a computing system is configured to identify one ormore defects of interest on a semiconductor wafer based on a compositeimage of a measured area. As depicted in FIG. 1, computing system 130receives a composite image 127 from multiple zone imaging detectordevice 120 and determines a location of one or more defects onsemiconductor wafer 101. Moreover, computing system 130 communicatessignals 128 indicative of the determined defect locations to memory 135.Memory 135 stores the defect locations for further analysis, reporting,etc.

In a further aspect, the measurement conditions associated withmeasurements of each location on the wafer are adjusted such that themeasurement conditions are different for the measurements performed byeach imaging detector. In some embodiments, the measurement conditionsare adjusted during the time between measurements by different imagingdetectors. This, in turn, enhances defect detection performance byincreasing the amount of signal information associated with eachmeasured location. Exemplary changes of measurement conditions includeenvironmental changes at the wafer under measurement and changes made tothe configuration of the inspection system, e.g., optical adjustments,focus adjustments, etc.

In some embodiments, illumination of a measured location within thefield of view of an imaging detector induces heating of the illuminatedstructures due to the incidence of high intensity photons at high flux.During the time between imaging by two different imaging detectors,(e.g., one millisecond), the induced heat spreads through the structuresunder measurement and stabilizes across the wafer surface. As a result,the temperature of the wafer at the measured location increases to ahigher temperature at the time of imaging of the measurement location bythe second imaging detector.

FIG. 1 depicts wafer conditioning device 140 located in close proximityto the surface of wafer 101. Wafer conditioning device 140 changes anenvironmental condition of the measured area between a time when thefirst image of the measured area is detected and a time when the secondimage of the measured area is detected. Command signals 141 arecommunicated from computing system 130 to wafer conditioning device 140.In response to command signals 141, wafer conditioning device 140changes the environmental condition (e.g., temperature, humidity,electric field, etc.) at the wafer surface.

In some embodiments, wafer conditioning device 140 is a heating devicethat increases the temperature of the wafer as the wafer passes. Theheating device may be a radiator that induces thermal heating of thewafer, a photon emitting device that induces thermal heating of thewafer, etc.

In some embodiments, wafer conditioning device 140 is a humidificationdevice that humidifies the local environment around the measured area.In some embodiments, the humidification device subjects the measuredarea to a flow of purge gas that includes a controlled amount of fillmaterial (e.g., water). A portion of the fill material (i.e., thecondensate) condenses onto the structures under measurement and fillsopenings in the structural features, openings between structuralfeatures, etc. The presence of the condensate changes the opticalproperties of the structure under measurement compared to a measurementperformed when the structure is devoid of any fill material.

In some embodiments, the humidification device includes a jet thatdelivers humid, clean air just in front of the field of view of imagingdetector 121A. The humid, clean air jet is isolated from the opticalelements to eliminate the potential for contamination of the opticalelements. In another embodiment, the humid, clean air jet delivershumid, clean air just in front of the field of view of imaging detector121B. In the humid environment, condensate quickly fills the waferstructure. Immediately after passing by the humidification device, thefilled structures are imaged by imaging detector 121B. In addition, themeasured structures are now subjected to a dry environment, along withheating due to illumination during imaging by imaging detector 121B.Evaporation of the fill material begins immediately after the filledstructures are subjected to the dry environment and continues during thetime the measured area moves from the field of view of imaging detector121B to the field of view of imaging detector 121A (e.g. approximatelyone millisecond) By the time the measured area is within the field ofview of imaging detector 121A, the fill material (e.g., water, alcohol,etc.) is evaporated, and the unfilled structures are imaged by imagingdetector 121A. For inspection scenarios where the fields of view of twoimaging detectors are adjacent to one another on the wafer, the wafer iscompletely filled when entering the field of view of the first imagingdetector (after treatment by the humidification device), mostly filledwhen leaving the field of view of the first imaging detector andentering the field of view of the second imaging detector, and mostly orcompletely unfilled when leaving the field of view of the second imagingdetector.

Filling structural features by condensate, for example, stronglyinfluences the measured images, especially at wavelengths in the vacuumultraviolet (VUV) range. Thus, defect detection based on a differentialimage, e.g., I₁-I₂, generated based on an image of filled structures,I_(i), and unfilled structures, I₂, enables improved high throughputwafer inspection. It is expected that the most significant imagedifferences occur at small structural gaps that are filled while beingimaged by one imaging detector, and mostly or completely dry (unfilled)when imaged by another imaging detector. Hence, the differential imageis especially sensitive to the absence of a small gap, an extra gap, ora significant change of gap size. Each of these defect scenarios areimportant targets of wafer inspection and can be difficult to detectwith existing wafer inspection systems.

In some embodiments, wafer conditioning device 140 is an electric fieldgenerator that introduces a local electric field on the wafer. Theapplied electric field may be constant or periodic, for example, in theradio frequency (RF) range or microwave range. In some embodiments, theelectric field generator includes electrodes incorporated in the housingof the objective lens. In some embodiments, the back side of the wafercan be used as an electrode. The presence of the local electric fieldchanges the reflectivity of structural features that are very sensitiveto electric field, and thus, enhances the sensitivity of differentialimages of those features.

In some embodiments, the electric field generator introduces an electricfield on the wafer within the field of view of imaging detector 121A,while no electric field is introduced on the wafer within the field ofview of imaging detector 121B. A differential image 127 is generatedbased on the difference between image signals associated with a measuredarea when imaged by imaging detector 121A and when imaged by imagingdetector 121B.

In some embodiments, the illumination and detection settings are thesame for images detected within the fields of view of imaging detectors121A and 121B. However, in another aspect, the illumination anddetection settings differ for images detected within the fields of viewof imaging detectors 121A and 121B. By way of example, differencesinclude any of optical intensity, wavelength spectrum, polarization,pupil image, or a combination thereof. The differences in opticalsettings enhance the sensitivity of differential images of features thatexhibit sensitivity to different optical measurement conditions.

In some embodiments, the illumination optics 112 are configured todirect light having a relatively narrow wavelength band to the specimen(e.g., nearly monochromatic light or light having a wavelength range ofless than about 20 nm, less than about 10 nm, less than about 5 nm, oreven less than about 2 nm). Therefore, if the light source is abroadband light source, the illumination optics 112 may also include oneor more spectral filters that may limit the wavelength of the lightdirected to the specimen. The one or more spectral filters may bebandpass filters and/or edge filters and/or notch filters. In someembodiments, each of optical zones 112A and 112B include differentspectral filters and generate illumination light 113A and 113B havingdifferent spectral characteristics (e.g., different wavelength ranges).

In another aspect, the focus offset is adjusted to be different forimages detected within the fields of view of imaging detectors 121A and121B. In some embodiments, each imaging detector is located at adifferent focal plane. This approach may be implemented when eachimaging detector is packaged as a stand-alone detector device. In someembodiments, each imaging detector is located along a plane that istilted with respect to the focal plane. This approach may be implementedwhen each imaging detector is packaged as part of an integrated,multiple zone imaging detector device. In some embodiments, the waferitself is tilted with respect to the focal plane, such that the focusoffset associated with a particular measured area is different whenimaged within the field of view of detector 121A and when imaged withinthe field of view of detector 121B. Differential images of a measurementarea imaged at different focus offsets exhibit enhanced phase contrastand can also be used as the basis for an interferometry inspection mode.

Although, many changes in measurement conditions are visible indifferential images as described herein, in some examples, changes inmeasurement conditions are visible in composite images generated byaveraging. In these examples, a composite image is generated as anaverage of images detected by different imaging detectors and differenttimes, e.g., I₁+I₂/2. With a change of measurement conditions betweenimages, some image features may change because of the change ofmeasurement conditions and will be blurred by averaging. However, otherimage features may not change and will be enhanced.

Any of the illumination and collection optics described herein may be alens, a compound lens, or any appropriate lens known in the art.Alternatively, any of the illumination and collection optics describedherein may be a reflective or partially reflective optical component,such as a mirror. In addition, although particular collection angles areillustrated in FIG. 1, it is to be understood that the collection opticsmay be arranged at any appropriate collection angle. The collectionangle may vary depending upon, for example, the angle of incidenceand/or topographical characteristics of the specimen.

Each imaging detector generally functions to convert the scattered lightinto an electrical signal, and therefore, may include substantially anyphotodetector known in the art. However, a particular detector may beselected for use within one or more embodiments of the invention basedon desired performance characteristics of the detector, the type ofspecimen to be inspected, and the configuration of the illumination. Forexample, if the amount of light available for inspection is relativelylow, an efficiency enhancing detector such as a time delay integration(TDI) camera may increase the signal-to-noise ratio and throughput ofthe system. However, other detectors such as charge-coupled device (CCD)cameras, photodiode arrays, phototube and photomultiplier tube (PMTs)arrays may be used, depending on the amount of light available forinspection and the type of inspection being performed. The term “imagingdetector” is used herein to describe a detector having a sensing area,or possibly several sensing areas (e.g., a detector array or multi-anodePMT). Regardless of number, the sensing areas of an imaging detectorgenerate image signals independent from any other imaging detector.

Wafer inspection system 100 also includes various electronic components(not shown) needed for processing the scattered signals detected byimaging detectors 121A and 121B. For example, wafer inspection system100 may include amplifier circuitry to receive output signals from anyof detectors 121A and 121B, and to amplify those output signals by apredetermined amount and an analog-to-digital converter (ADC) to convertthe amplified signals into a digital format suitable for use withinprocessor 131.

In general, processor 131 is configured to detect features, defects, orlight scattering properties of the wafer using composite images. Thecomposite images are generated based on the image signals produced byimaging detectors 121A and 121B. The composite images may be generatedby detector 120 as described hereinbefore, or may be generated bycomputing system 130 based on the image signals produced by imagingdetectors 121A and 121B. In this manner, the functionality of arithmeticprocessing unit 132 may by implemented by computing system 130 in someembodiments. The processor may include any appropriate processor knownin the art. In addition, the processor may be configured to use anyappropriate defect detection algorithm or method known in the art. Forexample, the processor may use a die-to-database comparison or athresholding algorithm to detect defects on the specimen.

In addition, wafer inspection system 100 may include peripheral devicesuseful to accept inputs from an operator (e.g., keyboard, mouse,touchscreen, etc.) and display outputs to the operator (e.g., displaymonitor). Input commands from an operator may be used by processor 131to adjust threshold values used to control illumination characteristics,collection characteristics, wafer conditioning characteristics, etc. Theresulting characteristics may be graphically presented to an operator ona display monitor.

Wafer inspection system 100 can use various imaging modes, such asbright field, dark field, and confocal. For example, in one embodiment,multiple zone imaging detector device 120 generates a bright fieldimage. As illustrated in FIG. 1, some amount of light scattered from thesurface of wafer 101 at a narrow angle is collected by objective lens129. This light passes back through objective lens 129, collectionoptics 118A-B, and onto multiple zone imaging detector device 120. Inthis manner bright field images are generated by multiple zone imagingdetector device 120.

In some embodiments, an aperture or Fourier filter, which can rotate insynchronism with the wafer, is placed at the back focal plane ofobjective lens 129. Various imaging modes such as bright field, darkfield, and phase contrast can be implemented by using differentapertures or Fourier filters. U.S. Pat. Nos. 7,295,303 and 7,130,039,which are incorporated by reference herein, describe these imaging modesin further detail. In another example, multiple zone imaging detectordevice 120 generates dark field images by imaging scattered lightcollected at larger field angles. In another example, a pinhole arraythat matches the layout of the illumination spot array can be placed infront of each imaging detector to generate a confocal image. U.S. Pat.No. 6,208,411, which is incorporated by reference herein, describesthese imaging modes in further detail. In addition, various aspects ofwafer inspection system 100 are described in U.S. Pat. Nos. 6,271,916and 6,201,601, both of which are incorporated herein by reference.

In the embodiment illustrated in FIG. 1, wafer positioning system 102moves wafer 101 under a stationary microscope. Wafer positioning system102 includes a wafer chuck 103, motion controller 104, a X-translationstage 105, and a Y-translation stage (not shown). Wafer 101 is supportedon wafer chuck 103. As illustrated in FIG. 1, translation stage 105translates the wafer 101 in the X-direction at a specified velocity,V_(X), and the Y-translation stage translates the wafer 101 in theY-direction. Motion controller 104 coordinates the movements of wafer101 by the X and Y translation stages to achieve the desired stepping orscanning motion of wafer 101 within wafer inspection system 100.

As illustrated in FIG. 1, a single primary illumination source 110supplies the illumination energy for both the illumination beams. Insome embodiments, illumination source 110 may be a broadband source. Thebroadband light may be separated into different wavelength bandssupplied to different illumination optics to generate illumination beamsof different wavelength. Similarly, light collected from the wafersurface may be separated into different wavelength bands and directed todifferent imaging detectors. As discussed herein, reflected andscattered light collected from the wafer surface may be associated witheither illumination beam based on the field of view of each imagingdetector. However, distinguishing between reflected and scattered lightassociated with each illumination beam may also be based on thewavelength of the collected light when different wavelength light isused to generate each illumination beam. In this manner, imagingdetector signals originating from different illumination beams may bedistinguished even when the fields of view of each imaging detector arelocated close together on the wafer surface.

FIG. 5 illustrates a method 200 of performing imaging based measurementsof a measured area by multiple imaging detectors in at least one novelaspect. Method 200 is suitable for implementation by an inspectionsystem such as inspection system 100 illustrated in FIG. 1. In oneaspect, it is recognized that data processing blocks of method 200 maybe carried out via a pre-programmed algorithm executed by one or moreprocessors of computing system 130, or any other general purposecomputing system. It is recognized herein that the particular structuralaspects of system 100 do not represent limitations and should beinterpreted as illustrative only.

In block 201, a first set of signals is generated indicative of a firstimage of a measurement location of a semiconductor wafer detected by afirst imaging detector.

In block 202, a second set of signals is generated indicative of asecond image of the measurement location of the semiconductor waferdetected by a second imaging detector. The first image of themeasurement location is detected at a first time and the second image ofthe measurement location is detected at a second time.

In block 203, a composite image is generated based on the first set ofsignals indicative of the first image of the measurement location andthe second set of signals indicative of the second image of themeasurement location. Generating the composite image involvesdetermining a difference between the first set of signals and the secondset of signals.

In block 204, one or more defects of interest on the semiconductor waferare identified based on the composite image of the measurement location.

It should be recognized that one or more steps described throughout thepresent disclosure may be carried out by a single computer system 130or, alternatively, a multiple computer system 130. Moreover, differentsubsystems of system 100, may include a computer system suitable forcarrying out at least a portion of the steps described herein.Therefore, the aforementioned description should not be interpreted as alimitation on the present invention but merely an illustration.

In addition, the computer system 130 may be communicatively coupled tothe hyperspectral detectors in any manner known in the art. For example,the one or more computing systems 130 may be coupled to computingsystems associated with the imaging detectors. In another example, theimaging detectors may be controlled directly by a single computer systemcoupled to computer system 130.

The computer system 130 of the system 100 may be configured to receiveand/or acquire data or information from the subsystems of the system(e.g., imaging detectors and the like) by a transmission medium that mayinclude wireline and/or wireless portions. In this manner, thetransmission medium may serve as a data link between the computer system130 and other subsystems of system 100.

Computer system 130 of system 100 may be configured to receive and/oracquire data or information (e.g., measurement results, modeling inputs,modeling results, reference measurement results, etc.) from othersystems by a transmission medium that may include wireline and/orwireless portions. In this manner, the transmission medium may serve asa data link between the computer system 130 and other systems (e.g.,memory on-board system 100, external memory, or other external systems).For example, the computing system 130 may be configured to receivemeasurement data from a storage medium (i.e., memory 132 or an externalmemory) via a data link. For instance, imaging data obtained using theimaging detectors described herein may be stored in a permanent orsemi-permanent memory device (e.g., memory 132 or an external memory).In this regard, the measurement results may be imported from on-boardmemory or from an external memory system. Moreover, the computer system130 may send data to other systems via a transmission medium. Forinstance, a composite image or an estimated parameter value determinedby computer system 130 may be communicated and stored in an externalmemory. In this regard, measurement results may be exported to anothersystem.

Computing system 130 may include, but is not limited to, a personalcomputer system, mainframe computer system, workstation, image computer,parallel processor, or any other device known in the art. In general,the term “computing system” may be broadly defined to encompass anydevice having one or more processors, which execute instructions from amemory medium.

Program instructions 134 implementing methods such as those describedherein may be transmitted over a transmission medium such as a wire,cable, or wireless transmission link. For example, as illustrated inFIG. 1, program instructions 134 stored in memory 132 are transmitted toprocessor 131 over bus 133. Program instructions 134 are stored in acomputer readable medium (e.g., memory 132). Exemplary computer-readablemedia include read-only memory, a random access memory, a magnetic oroptical disk, or a magnetic tape.

As described herein, the term “critical dimension” includes any criticaldimension of a structure (e.g., bottom critical dimension, middlecritical dimension, top critical dimension, sidewall angle, gratingheight, etc.), a critical dimension between any two or more structures(e.g., distance between two structures), and a displacement between twoor more structures (e.g., overlay displacement between overlayinggrating structures, etc.). Structures may include three dimensionalstructures, patterned structures, overlay structures, etc.

As described herein, the term “critical dimension application” or“critical dimension measurement application” includes any criticaldimension measurement.

As described herein, the terms “metrology system” or “inspection system”include any system employed at least in part to characterize a specimenin any aspect, including measurement applications such as criticaldimension metrology, overlay metrology, focus/dosage metrology, andcomposition metrology. However, such terms of art do not limit the scopeof the terms “inspection system” and “metrology system” as describedherein. In addition, the system 100 may be configured for measurement ofpatterned wafers and/or unpatterned wafers. The system may be configuredas a LED inspection tool, edge inspection tool, backside inspectiontool, macro-inspection tool, or multi-mode inspection tool (involvingdata from one or more platforms simultaneously), and any other metrologyor inspection tool that benefits from the imaging techniques describedherein.

Various embodiments are described herein for a semiconductor waferinspection system that may be used for measuring a specimen within anysemiconductor processing tool (e.g., an inspection system or alithography system). The term “specimen” is used herein to refer to awafer, a reticle, or any other sample that may be processed (e.g.,printed or inspected for defects) by means known in the art.

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples include, butare not limited to, monocrystalline silicon, gallium arsenide, andindium phosphide. Such substrates may be commonly found and/or processedin semiconductor fabrication facilities. In some cases, a wafer mayinclude only the substrate (i.e., bare wafer). Alternatively, a wafermay include one or more layers of different materials formed upon asubstrate. One or more layers formed on a wafer may be “patterned” or“unpatterned.” For example, a wafer may include a plurality of dieshaving repeatable pattern features.

A “reticle” may be a reticle at any stage of a reticle fabricationprocess, or a completed reticle that may or may not be released for usein a semiconductor fabrication facility. A reticle, or a “mask,” isgenerally defined as a substantially transparent substrate havingsubstantially opaque regions formed thereon and configured in a pattern.The substrate may include, for example, a glass material such asamorphous SiO₂. A reticle may be disposed above a resist-covered waferduring an exposure step of a lithography process such that the patternon the reticle may be transferred to the resist.

One or more layers formed on a wafer may be patterned or unpatterned.For example, a wafer may include a plurality of dies, each havingrepeatable pattern features. Formation and processing of such layers ofmaterial may ultimately result in completed devices. Many differenttypes of devices may be formed on a wafer, and the term wafer as usedherein is intended to encompass a wafer on which any type of deviceknown in the art is being fabricated.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A semiconductor wafer inspection systemcomprising: a first imaging detector that generates a first set ofsignals indicative of a first image of a measurement location of asemiconductor wafer detected by the first imaging detector; a secondimaging detector that generates a second set of signals indicative of asecond image of the measurement location of the semiconductor waferdetected by the second imaging detector, wherein the first image of themeasurement location is detected at a first time and the second image ofthe measurement location is detected at a second time that is temporallyseparated from the first time; an image combining device that generatesa composite image based on the first set of signals indicative of thefirst image of the measurement location and the second set of signalsindicative of the second image of the measurement location; a waferpositioning system operable to move the wafer in a scanning motion suchthat a field of view of the first imaging detector and a field of viewof the second imaging detector move across the semiconductor wafer alongan inspection path, wherein the field of view of the first imagingdetector is located ahead of the field of view of the second imagingdetector along the inspection path; and a computing system configured toidentify one or more defects of interest on the semiconductor waferbased on the composite image of the measurement location.
 2. Thesemiconductor wafer inspection system of claim 1, wherein the first timeand the second time are temporally separated by at least 100microseconds and no more than 10 milliseconds.
 3. The semiconductorwafer inspection system of claim 1, wherein the first imaging detectorand the second imaging detector are integrated into a multiple zonedetector device.
 4. The semiconductor wafer inspection system of claim1, further comprising: a wafer conditioning device that changes anenvironmental condition of the measurement location between a time whenthe first image of the measurement location is detected and a time whenthe second image of the measurement location is detected.
 5. Thesemiconductor wafer inspection system of claim 4, wherein theenvironmental condition is an electrical field at the surface of thesemiconductor wafer, a humidity at the surface of the semiconductorwafer, a temperature at the surface of the semiconductor wafer, or acombination thereof.
 6. The semiconductor wafer inspection system ofclaim 1, wherein a focus offset at the semiconductor wafer associatedwith the first image is different from a focus offset at thesemiconductor wafer associated with the second image.
 7. Thesemiconductor wafer inspection system of claim 1, further comprising:one or more illumination sources configured to generate an amount ofoptical radiation; and one or more optical elements configured toreceive the amount of optical radiation and direct a first amount ofincident illumination light to the semiconductor wafer within a field ofview of the first imaging detector and a second amount of incidentillumination light to the semiconductor wafer within a field of view ofthe second imaging detector.
 8. The semiconductor wafer inspectionsystem of claim 7, wherein the first amount of incident illuminationlight and the second amount of incident illumination light differ fromone another in optical intensity, wavelength spectrum, polarization,pupil image, or a combination thereof.
 9. The semiconductor waferinspection system of claim 1, wherein the first and second imagingdetectors are charge-coupled device (CCD) detectors or complementarymetal on silicon (CMOS) detectors.
 10. The semiconductor waferinspection system of claim 1, wherein the first and second imagingdetectors operate in a time delay integration (TDI) mode.
 11. Thesemiconductor wafer inspection system of claim 1, wherein the generatingof the composite image involves determining a difference between thefirst set of signals and the second set of signals on a pixel by pixelbasis.
 12. A semiconductor wafer inspection system comprising: a firstimaging detector that generates a first set of signals indicative of afirst image of a measurement location of a semiconductor wafer detectedby the first imaging detector; a second imaging detector that generatesa second set of signals indicative of a second image of the measurementlocation of the semiconductor wafer detected by the second imagingdetector, wherein the first image of the measurement location isdetected at a first time and the second image of the measurementlocation is detected at a second time; an image combining device thatgenerates a composite image based on the first set of signals indicativeof the first image of the measurement location and the second set ofsignals indicative of the second image of the measurement location,wherein the generating of the composite image involves determining adifference between the first set of signals and the second set ofsignals; a wafer positioning system operable to move the wafer in ascanning motion such that a field of view of the first imaging detectorand a field of view of the second imaging detector move across thesemiconductor wafer along an inspection path, wherein the field of viewof the first imaging detector is located ahead of the field of view ofthe second imaging detector along the inspection path; and a computingsystem configured to identify one or more defects of interest on thesemiconductor wafer based on the composite image of the measurementlocation.
 13. The semiconductor wafer inspection system of claim 12,wherein the first time is temporally separated from the second time byat least 100 microseconds and no more than 10 milliseconds.
 14. Thesemiconductor wafer inspection system of claim 12, wherein thedifference between the first set of signals and the second set ofsignals is determined on a pixel by pixel basis.
 15. The semiconductorwafer inspection system of claim 12, wherein the first imaging detectorand the second imaging detector are integrated into a multiple zonedetector device.
 16. The semiconductor wafer inspection system of claim12, further comprising: a wafer conditioning device that changes anenvironmental condition of the measurement location between a time whenthe first image of the measurement location is detected and a time whenthe second image of the measurement location is detected.
 17. Thesemiconductor wafer inspection system of claim 12, wherein a focusoffset at the semiconductor wafer associated with the first image isdifferent from a focus offset at the semiconductor wafer associated withthe second image.
 18. The semiconductor wafer inspection system of claim12, wherein a first amount of incident illumination light directed tothe semiconductor wafer within a field of view of the first imagingdetector differs from a second amount of incident illumination lightdirected to the semiconductor wafer within a field of view of the secondimaging detector in optical intensity, wavelength spectrum,polarization, pupil image, or a combination thereof.
 19. A methodcomprising: generating a first set of signals indicative of a firstimage of a measurement location of a semiconductor wafer detected by afirst imaging detector; generating a second set of signals indicative ofa second image of the measurement location of the semiconductor waferdetected by a second imaging detector, wherein the first image of themeasurement location is detected at a first time and the second image ofthe measurement location is detected at a second time; generating acomposite image based on the first set of signals indicative of thefirst image of the measurement location and the second set of signalsindicative of the second image of the measurement location, wherein thegenerating of the composite image involves determining a differencebetween the first set of signals and the second set of signals; movingthe semiconductor wafer in a scanning motion such that a field of viewof the first imaging detector and a field of view of the second imagingdetector move across the semiconductor wafer along an inspection path,wherein the field of view of the first imaging detector is located aheadof the field of view of the second imaging detector along the inspectionpath; and identifying one or more defects of interest on thesemiconductor wafer based on the composite image of the measurementlocation.